If someone could make an interface that would be generic they might start considering it...
I just built a prototype circuit yesterday. It still needs some work, but it looks promissing.
But all in all... very very hard to do in a mannor that would work for all / most games. (basic forced feedback really)
I looked at the code for the Widel hack and it doesn't appear that difficult:
static WRITE16_HANDLER( term2_gun1_w )
{
wms_scratch_ram[0x000a073]=data;
if (((data & 0xFF) == 0xEF) & ((wms_scratch_ram[0x000a231] & 0xff00) == 0x0100) )
{
set_led_status(0, 1);
}
else
{
set_led_status(0, 0);
};
}